linux-kernel - Re: [PATCH] rcutorture: Fix rcu_torture_pipe_update_one()/rcu_torture

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Date: Wed, 6 Mar 2024 12:20:12 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, linke li <lilinke99@...com>, joel@...lfernandes.org, 
	boqun.feng@...il.com, dave@...olabs.net, frederic@...nel.org, 
	jiangshanlai@...il.com, josh@...htriplett.org, linux-kernel@...r.kernel.org, 
	mathieu.desnoyers@...icios.com, qiang.zhang1211@...il.com, 
	quic_neeraju@...cinc.com, rcu@...r.kernel.org
Subject: Re: [PATCH] rcutorture: Fix rcu_torture_pipe_update_one()/rcu_torture_writer()
 data race and concurrency bug

On Wed, 6 Mar 2024 at 11:46, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> That 'rtort_pipe_count' should be an atomic_t, and the "add one and
> return the old value" should be an "atomic_inc_return()-1" (the "-1"
> is because "inc_return" returns the *new* value).

Bah. I am lost in a twisty maze of operations, all the same.

One final correction to myself: if you want the old value, the nicer
thing to use is probably just "atomic_fetch_inc()".

It generates the same result as "atomic_inc_return()-1", but since we
do have that native "return old value" variant of this, let's just use
it.

So the rules are "atomic_op_return()" returns the new value after the
op, and "atomic_fetch_op()" returns the old value.

For some ops, this matters more than for others. For 'add' like
operations, it's you can deduce the old from the new (and vice versa).

But for bitwise ops, only the 'fetch" version makes much sense,
because you can see the end result from that, but you can't figure out
the original value from the final one.

And to *really* confuse things, as with the memory ordering variants,
we don't always have the full complement of operations.

So we have atomic_fetch_and() (returns old version) and atomic_and()
(doesn't return any version), but we don't have "atomic_and_return()"
because it's less useful.

But for 'inc' we have all three.

                        Linus